1. Field of the Invention
The present invention relates to a pixel circuit and its driving method and, more particularly, to an active matrix OLED pixel circuit and its driving method suitable for compensating transistor threshold voltage and OLED voltage.
2. Description of Related Art
Driving transistors of active matrix OLED (AMOLED) can be classified to P-type transistors and N-type transistors according to its back plate manufacture technologies. Please refer to FIG. 1 and FIG. 2, which are prior P-type and N-type driving circuits of AMOLED, respectively. As shown in FIG. 2, for the N-type driving circuit, there is a problem in that the threshold voltage of the N-type transistor may be shifted. This threshold voltage shift is caused by the generation of degradation due to the manufacture variation and long-time operation, resulting in being unable to output a current same as the initial current and thus producing mura or brightness decay. Moreover, due to that the OLED is operated for a long time, the operating voltage is increased following the increase of the time. Thus, to solve the aforementioned problem, an N-type compensation circuit is proposed. With reference to both FIG. 3 and FIG. 4, there are a schematic diagram of N-type AMOLED compensation driving circuit and a timing diagram of N-type compensation driving circuit. As shown in FIG. 3 and FIG. 4, it can be seen that the number of components (6T2C) in the pixel circuit design is too many and the driving signals (Sn, Sn′, En, Xen) become too complicated, resulting in being unable to satisfy the requirements of high precision and high aspect ratio.
Therefore, it is desirable to provide an improved pixel circuit and its driving method, in which N-type driving transistors are used to drive the OLED, and combined with a plurality of transistors and capacitors to compensate the threshold voltage of the N-type transistor and the voltage of the AMOLED, so as to satisfy the requirements of high precision and high aspect ratio.